Catalytic hydrogenation of hydroaromatic carboxylic acids and their esters



Patented Jan. 18,1938

oFFrca CATALYTIC HYDROGENATION F HYDRO- AROMATIC CARBOXYLIC ACIDS AND THEIR ESTERS Wilbur A. Lazier, Marshallton, Del., assignor to E. I. du Pont de Nemours & Company,'Wilrmngton, Del., a corporation of Delaware No Drawing. Application November 21, 1934, $3521] No. 754,184. In Great Britain April 17,

14 Claims. (o1. 260-153) This invention relates to catalytic processes for the production of cyclic alcohols, esters, and ethers. More particularly, it relates to processes for the catalytic reduction by means of elementary-hydrogen of homocyclic hydroaromatic acids,

their esters, and their anhydrides to the corresponding alcohols. Specifically; the invention relates to-the hydrogenation of hexahydrophthalic acids and their derivatives, and to the use of certain catalysts particularly well suited to these reactions.

This application is a continuation in part of co-pending applications Serial Nos. 584,573 and 584,574, filed January 2, 1932; Serial No. 470,238, filed July 23, 1930; Serial No. 445,224, filed April 17, 1930; and Serial No. 690,568, filed September 22, 1933.

For many years the only known methods for the reduction of carboxylic acids, esters, and their anhydrides to the corresponding alcohols were by purely chemical means involving the consumption of expensive reducing agents. The most successful procedure was that outlined by Bouveault and Blanc (Chem. Zentr., 1904,,II, 184; 1905, II, 1700). This process involves preparing an ester of the acid to be reduced and the use of metallic sodium and absolute alcohol as the reducing agent. Thus, it has been possible to prepare alcoholic derivatives of the simple aliphatic carboxylic acids. This method, however, is so costly as to render its use prohibitive for the manufacture of various alcoholic products which might otherwise be very useful in the arts.

By suitable modifications of processes fully described in the copending specifications to which reference has already been made, it has now be-- come possible to realize on a commercial scale a technically and economically successful catalytic hydrogenation of homocyclic hydroaromatic acids, their esters, and their anhydrides, whereby alcohols are .formed which correspond in the number of carbon atoms to the acids or acid derivatives subjected to the hydrogenation treatment. Other products such as the corresponding 45 saturated hydrocarbons, ethers, and esters of the newly formed alcohols may also be prepared in this way by minor variations in the procedure,

but the invention is primarily concerned with the production of alcohols which are the intermediate products betweenthe esters and the cor responding hydrocarbons resulting from exhaus-.

filed January 2, 1932, there is contained a description of the successful hydrogenation of carboxylic ring compounds such as carboxylic acids of the homocyclichydroaromatic series and their compound with a suspended nickel catalyst in the presence of gaseous hydrogen under a pressure slightly in excess of atmospheric pressure. In the processes of the prior art, the temperatures employed are usually 150",200'C."and are never greater than 225 C., while the pressures customarily used are only slightlyin excess of atmospheric pressure.

This invention has as an object the provision of processes for the conversion of' hydroaromatic carboxylic acids, their esters, and their anhydrides to the corresponding alcohols and other products. A'further object of the invention is the provision of processes for the preparation oi the reduction products of hexahydrophthalates, benzoates, and toluates. A further object is the preparation of hexahydroaromatic alcohols including cyclohexyl alkanols and hexahydrophthalyl alcohols. A still further object is the preparation of hexahydrophthalide, hexahydrophthalyl ether, and methylcyclohexyl carbinol. Other objects will appear hereinafter.

These objects are accomplished by the follow ing invention wherein homocyclic hydroaromatic i boxylic compound such as'diethyl hexahydrophthalate and hydrogen into intimate contact with a suitable alcohol-forming catalyst at relatively'high temperatures and pressures, It has been found, in accordance with the present invention, that organic carboxylic compounds-in which:

the carboxylic group is attached directly to a saturated or partially saturated ring ,of carbon atcatalyst is preferably a composition containing.

copper either in the elementary form or combined with'oxygen as a lower oxide. Other hydrogenating metal oxides may be employed in conjunction with copper, or suitablecatalyst sup- 1 ports such as kieselguhr, silica gel, and activated caib'on may be used. In another modification of the process, the hydroaromatic carboxylic compounds and hydrogen are passed under high pres sures and elevated temperatures over mixedhy drogenation catalysts containing substantial quantities of difiicultly reducible oxides of hydrogenating metals prepared in a suitable granular form and held in place in a pressure-resisting tube. Contrary to expectation, it has been found that under high hydrogen pressures hydroaromatic acids and their derivatives are much less susceptible to decomposition by heat than would be supposed from their behavior when heated in air. Under reducing conditions and in the presence of a suitable catalyst the decomposition, if such it may be'termed, takes place in a controlled manner and with the absorption of hydrogen and'the production of the corresponding alcohols.

The following examples are illustrative-of some of the methods that may be employed in. the practice of the invention:

Example 1 f A hydrogenation catalyst was prepared as follows: 23 g. of cadmium nitrate, 24 g. of copper nitrate, and 245g. of zinc nitrate was dissolved in 500 cc. of water and mixed at ordinary temperature with an equal volume of water containing 126 g. ofammoniumbichromate and 75 cc. of 28 per cent ammonium hydroxide. After stirring,

the mixture was exactly neutralized with addi- Twenty-five cc. of the mixed chromite catalyst.

prepared as described above was loaded into an alloy steel reaction vessel capable of being heated Y and withstanding high pressures. The tube was fitted with a preheater, a pump for injecting liquid at a constant rate, a T-connection for introducing hydrogen under pressure, a suitable condenser and trap for separating liquid products,

andexit control valve.

.The-diethyl esterof hexahydrophthalic acid was pumped overthe catalyst at the rate of cc. per hour together with an amount or hydrogen equal to about twenty. times that necessary for complete reduction of the ester. The temperature was maintained at 385 C. and the hydrogen pressure at 2900 pounds per square inch. The ester was converted to the vapor phase in a'p'reheater before passing into the autoclave.

After leaving the autoclave; the liquid products formed by hydrogenation were separated from ,the hydrogen gas stream by condensation., Four hundred grams oi the crude product yielded upon ortho-substituted ester.

fractional distillation: 136 grams of 2-methyl cyclohexyl carbinol, 32 grams of hexahydrophthalyl ether, and 7 grams of hexahydrophthalyl alcohol.

Example 2 A copper chromite catalyst was prepared by igniting copper ammonium chromate and extract.- ing it twice with dilute acetic acid. The resulting copper chromite powder was employed 'for hydrogenation without further treatment.

A high pressure autoclave equipped with an agitator was charged with 4000 grams of the ethyl ester of hexahydro-ortho-phthalic acid and 320 grams of the copper chromite hydrogenation catalyst prepared as described above. Compressed hydrogen was introduced until a pressure of 3000 pounds per square inch was attained. The contents of the autoclave were heated and agitated for 11 hours at a temperature of 255 C., the hydrogen pressure beingmaintained by introducing a fresh supply of gas as needed. Hydrogen was rapidly absorbed as evidenced by the steady drop in pressure.

A determination of the saponification number of the reaction products indicated a 79.5 per cent conversion of the diethyl hexahydro-orthophthalate. The reaction products were saponified with alcoholic potassium hydroxide and the neutral fraction was extracted with ether and distilled. The aqueous layer contained the potassium salt of o-hydroxymethylbenzoic acid and was acidified to regenerate hexahydrophthalid.

The following compounds were obtained in the quantities given: 313 grams of hexahydrophthalyl alcohol (boiling point C. at 2.5 mm.); 500 grams of hexahydrophth'alyl ether (boiling point 32-35 C. at-2.5 mm.); 300 grams of 2-methylcyclohexyl carbinol (boiling point 59-63 C. at 2.5 mm.); 360 grams of hexahydrophthalide (boiling point 124-126 C. at 10 mm.).

In carrying out the hydrogenation in a batch liquid phase process, as described above, in place of the diethylhexahydro-ortho phthalate there may be used under the same conditions of temperature and pressure the anhydride of hexahydro-ortho-phthalic acid prepared by hydrogenata ing the salt of phthalic acid in the nucleus with a nickel catalyst, liberating the freeaid, and heating to form the .anhydride. Methyl and other alkyl hexahydrophthalic acids, esters, etc, may also be hydrogenated by this process.

Erample 3 The hydrogenation of diethyl' hexahydroteree phthalate diilfers markedly from that of the ortho-ester in that practically no by-products such as the hexahydrophthalyl ether or methylhexahydrobenzyl alcohol are formed. This behavior suggests that ether formation by elimination of 1 water from the glycol does not take placein the para-compound as readily asin the caseof the Terephthalic acid was esterifled with ethyl alcohol to form the diethyl ester. This compound was hydrogenated in a stirring autoclave in the.

liquid phase with a reduced nickel on-kiesel'guhr catalyst at .a temperature of 200C. and aflhydrogen pressure of 2000 pounds. The yield ofdiethyl hexahydroterephthalate was almost quantitative. After removing the nickel catalyst theiester was subjected to carboxyl hydrogenation by the method indicated below.

A copper chromite ca'talystwas preparedas IOIIOWS: One'thousand-five hundred grams oI' copper nitrate dissolved in 4 liters of water was mixed with a solution containing 1000 grams of ammonium chromate in an equal volume of water. Ammonium hydroxide was added to neutralize the acidity developed during precipitation of the copper'ammonium chromate. The precipitate was washed by decantation, filtered, and dried, after which it' was ignited at a temperature of400 C. The resulting copper chromite powder was employed for hydrogenation without further treatment.

Two hundred and fifty grams of the hexahydroterephthalate was agitated with 20 g. of the copper chromite catalyst at a temperature of 255 C. and under a hydrogen pressure of 3000 pounds per square inch. The decrease in saponification value corresponded to a per cent hydrogenation of the ester to alcohols. An alkali insoluble fraction was fractionally distilled giving a prodcent and practically the entire product boiled constantly on redistillation. On the basis of the 90 per cent reduction in ester number, this corresponds to a 77.5 per cent yield on the raw material reacting.

I Example 4 A zinc-oxide-copper-kieselguhr catalyst for use in the hydrogenation of esters is prepared in the following manner: To a solution containing zinc nitrate and copper nitrate in equimolar proportions there isadded an excess of ammonium hy droxide. The precipitate formed at first redissolves in the excess of the reagent. Kieselguhr is then added to the solution to the extent of g. per mol. of metallic salts present and the whole charge is heated to 80 C. Atthis temperature air is passed thru the suspension until the excess ammonia is discharged. washed, filtered, and dried and reduced at 250- 300 C. in a streamof diluted hydrogen for about 10 hours.

A high-pressure autoclave is charged with 400 g. of the ethyl ester of hexahydro-ortho-toluic acid and 40 g. of'the zinc oxide-copper-kieselguhr catalyst prepared. as described above. Compressed hydrogen is introduced until a pressure of about 250 atmospheres is reached. The contents of the autoclave are heated 6 hours with stirring at a. temperature of 325 C., a high hydrogen pressure being maintained by introduction of a fresh supply of gas. Hydrogen is rapidly abduced to cyclohexyl carbinol, and the hydrogento the corresponding alcohols.

Example 5 A copper chromite catalyst is prepared by ignitated naphthoates and hydrogenated anthroates ing copper ammonium chromate and briquetting the residualchromite composition. A solution of 500 g; of hexahydrobenzoic acid dissolved in two liters of hot absolute alcoholjs passed over this catalyst together with an amount of hydrogen The precipitate is equal to 20 times that necessary for complete reduction of the carboxylic acid. The temperature is maintained at 385? C. and the pressure on the reaction system is held between 2500 and 3000 lbs./sq. in. The rate of flow of the acid solution is 150 cc./hour. The conditions for the reaction are controlled and the product collected as de scribed in Example 1.

The reaction products are characterized'by a low acid and saponification number indicating substantial conversion of the free acid to alcohol. After evaporation of the ethanol, cyclohexyl carbinol is obtained as the major product of the reaction.

The process is generally applicable to homocyclic hydroaromatic carboxylic acids, their esters, and anhydrides, i. e., to the acyl radicals of these acids but the acids, etc., having no more than 2 carbocyclic rings form a preferred group.

Example 6 Two hundred grams of ethyl phenyiacetate were converted into the corresponding ethyl cyclohexylacetate by hydrogenation in the presence of 20 g. of reduced nickel-on-kieselguhr at a temperature of C. and a pressure of 2000 lbs. per sq. in. The ethyl ester of cyclohexylacetic acid was isolated in a pure condition by fractional distillation. It was then subjected to carboxylic reduction by hydrogenation at higher temperaturm.

and pressures in the presence of a barium-copperchromite catalyst. The compound catalyst for this purpose was prepared by dissolving 26 g. of barium nitrate and 218 g. of cupric nitrate in 800 cc. of warm water. A multiple chromate precipitate was formed by the addition with stirring oi'a solution prepared by dissolving 126 g. of ammonium bichromate in 600 cc. of water and adding cc. of 28 per cent ammonium'hydroxide.

The precipitate was washed, filtered, dried, 18f nited at 400 C. and extracted once or twice with 10 per cent acetic acid, and again washed and dried. Two hundred grams of ethyl cyclohexylacetate were then hydrogenated in the presence 0! 20 g. of the Cu-Ba-Cr catalyst prepared as described above. The hydrogen pressure used was about 3000. lbs. per sq. in. and the temperature about 255 C. The cmde product was filtered, and subjected to continuous extraction whereby a 43 per cent yield of cyclohexylethanol was obtained.

In a similar manner .cyclohexylpropanol has been prepared, starting with ethyl cinnamate. In this case both the oleflne bond and the arcmatic nucleusare attacked by the hydrogenation with nickel catalyst to yield the corresponding ethyl eyclohexylpropionate. This compound was thereafter hydrogenated to gamma cyclohexylpropanol according to the methods of this invention as outlined in the latter part of the above example. 1

Although certain definite conditions of operation such as temperature, pressure and time of contact of the material treated with the catalyst have been indicated in the above examples, it will be apparent that these factors may be varied within wide limits within the scope of the present invention. The catalytic reduction of hydroaromatic acids,- their esters, and their anhydrides to alcohols or glycols requires the use of temperatures-and pressures appreciably higher than customarily employed for other hydrogenation reactions. The temperature may range irom above 200 C. up to 500 C. The preferred temperature range is 240-400 C., depending somewhat on the catalyst compontion selected and the method.

used for carrying out a given reaction. The success of .the process also depends on the use of elevated pressures ,in excess of 10 atmospheres,

while the preferred pressure is -400 atmos pheres. The maximum pressure which can be used is limited only by the strength of the reaction apparatus.

Where a flow process is used, the ratio of hydro-' gen to the carboxylic acid or the acid derivatives may be varied over a wide range; but the use of a substantialmolecular excess of hydrogen,

e. g., a ratio of about 2 to 10 mols or more of hydrogen per mol. of the material undergoing ployed at the expense of slightly lower rates of conversion.

Whereas the critical factors and inventive steps in the hydrogenation of hydroaromatic 'carboxylic compounds to alcohols are the use of high temperatures and pressures, it necessarily follows that suitable catalysts may-be selected from among a number of different hydrogenating metals and oxides. Mild hydrogenating catalysts suchas metallic copper and zinc oxide which are well known to be suitable for the synthesis of methanol from-carbon monoxide and hydrogen are in general also suitable catalysts for the production of hydroaromatie alcohols. other hand there arecertain very energetic catalysts such as metallic nickel and. iron which are known to'catalyze theformation of hydrocarbons from oxides of carbon and hydrogen. These ferrous metalcatalysts, when employed in the hydrogenation of hydroaromatic acids and their esters tend to .c'arry the reaction too far with the formation of hydrocarbons. Therefore if the hydrogenation is to befoperated for the productionof alcohols to the substantial-exclusion of hydrocarbons it is preferable to select as the catalyst a composition comprising a member of the group of non-ferrous hydrogenating metals such as copper, tin, silver, cadmium, zinc, lead, their oxides and chromites, and oxides and chromites of manganese, and magnesium. lllsoxide, zinc'oxide, magnesium oxide, or chromium oxide. The above mentioned mild-acting catalysts may be termed the alcohol-forming catalysts to disting'uishthem from-the more energetic hydrocarbon-forming elements of the platinum and ferrous metal groups; 'Elementary nickel, cobalt, and iron when suitably supported on kieselguhrmay be. used to effect the reduction of carboxylic'compounds with hydrogen, but in I these cases the product contains besides alcoholic bodies a preponderance of hydrocarbons, and this disadvantage in most cases ,will prove so, v serious as to preclude-the use of these catalysts unless the hydrocarbons themselves are the" de-, sired end products. Non-ferrous metal, nonv o' v o latinum metal, hydrogenating catalysts are a p -B- -c1 !.).g-ox rue on R 0-t!-n+ cH,).-6 ort therefore preferred.

Catalysts suitable for use in the liquid phase batch method of hydrogenation. are preferably On the prepared in a powder form. t The preferred catalyst forthis purpose is usually acopper chromite,

prepared by igniting a double copper ammonium vchromate to .its spontaneous decompositiontemperature as described in U. S. Patent 1,746,783. Many modifications of this procedure have been practiced involving the use of acid extraction, hydrogen reduction, and the use of a supplementary support such as kieselguhr but these are modifications in degree only. The essential fea- -.ture is the use of copper oxide intimately associated or combined with chromium sesquioxide and the chromite method of preparation is a convenient method for effecting the desired association.

The method," however, is not limited to copper,

rugged character and the ease with which they may be shaped into hard granules for loading "into 'stationary apparatus. By the term diflicultly reducible is meant that the oxides are not substantially reduced to metal by prolonged exposure in a state of purity to the action .of hyfor use as catalysts in the hydrogenation of hydroaromatic carboxylic compounds are zinc ox- .drogen at atmospheric pressure and-at a tem- -perature of 400-450'C. I Such oxides suitable ide, manganese oxide,. and magnesium oxide.

These oxides may be employed either alone or in combination with 'each other or with other metals or oxides which have a promoting action. Preferably the diflicuitly reducible hydrogenating oxides also are prepared in the form of chromites as already indicated in the examples.

Hydroaro'matic acids and their derivatives are particularly susceptible to conversion to hydrocarbons, and ,it has been found that this undesirable side reaction may be largely prevented by employing a mild inorganic base added to the hydrogenation catalyst. For example, products having much higher hydroxyl valuesare obtained if a little magnesia, zinc oxide, lime, or barium hydroxide is added to the catalyst or to the reaction system. Preferably the alkali earth bufier is incorporated into the catalyst at the time of itsprecipitation.

The processes of. the present invention are ap- I plicable generally to a wide variety of organic.

acids and acid'derivatives containing ring structures. In particular they are applicable to carboxylic' compounds belongingtothe class characterized by the term homocyclic hydroaromatic acids. Some important membersof this group are hexahydrobenzoic acid, the various hexahydrohydroxybenzoic acids such as hexahydro-' salicylic. acid, 1-, 2-, and 3- methylhexahydrobenzoic acids, hexahydro-ortho iso-,- and tenphthalic acids; hydrogenated anisic, naphthoic,

pounds represented by the structure:

etc., where R is a hydroaromatic ring. a. may be any-value from 0 to 10 inclusive, and R is hy- The invention is applicable to the type of comdrogen, alkyl, or acyl. For hydroaromatic carboxylic compoundshaving the carboxyl attached directly to the ring, of course :r=0. In thecase of ethyl cyclohexylacetate and cyclohexylpropionate, a:=1 and 2, respectively.

\The carboxylic compounds may be employed in the form of theiree acid, or as an ester or anhydride of. the acid. v I Many variations and modifications of the processes described may be employed to obtain the desired variations in the kind and quantity of hydrogenated products. In place ofhexahydrophthalic acid or diethyl hexahydrophthalate or hexahydroterephthalate as above disclosed, other esters such as the butyl, cyclohexyl, propyl, amyl, and benzyl, and the glycerol and glycol -esters may be employed. The methyl ester may be employed, but under some conditions it gives rise to undesirable by-products. Hexahydrophthalic anhydride may also be used; The corresponding derivatives of hexahydroisophthalic and hexahydroterephthallc acids may be used. The process is also applicable to the hydrogenation of substituted hexahydrophthalic, hexahydro-iso-' phthalic and. hexahydroterephthalic acids such as the -methyland ethyl-hexahydrophthalic acids. and their'derivatives. The acid or acid derivative may be in the liquid'or vapor state and may be hydrogenated in solution in v .a' suitable inert solvent.

The above description and examples are intended to he illustrative only. Any modification class consisting of homocyclic hydroaromatic carboxylic acids and their esters, at a pressure of at least 10 atmospheres and at a temperature in excess of 200 C.

2. The process which comprises catalytically h'ydrogenating the acyl radical of a homocyclic. hydroaromatic carboxylic acid to a homocyclic hydroaromatic alcoholat a pressure of at least 10 atmospheres and at a temperature in excess of 200 C. I

3. The process which comprises catalytically hydrogenating the acyl radical of a homocyclic 'hydroaromatic carboxylic acid having not more than two carbocyclicring's to a homocyclic hydroaromatic alcohol at a pressure of at least 10 atmospheres and at a' temperature in excess of 200C. j I

4. The process acyl radical of a compound selected from the which comprises catalyticallyhydrogenating a homocyclic hydroaromatic car-I boxylic acid to a homocyclic .hydroaromatic alcohol at a pressure of at least 10 atmospheres and at a temperature in excess of 200 C.

5. The process which comprises catalytically hydrogenating' a homocyclic hydroaromatic carboxylic acid having not more than two carbocyclic rings to a. homocyclic hydroaromatic alcohol at a'pressure of at least 10 atmospheres and at a temperature in'excess of 200 C.

6. The process which comprises catalytically hydrogenating a homocyclic hydroaromatic carboxylic acid having not more than two carbocyclic rings to a homocyclic hydroaromatic alcohol at a pressure of at least 10 atmospheres and at a temperature of 300-400 C.

7. The process which comprises catalytically hydrogenating a homocyclic hydroaromatic carboxylicacid having not more than two carbocyclic rings to a homocyclic hydroaromatic alcohol at a pressure of at least 10 atmospheres and at a temperature of 300-400 C. .in the presence of a hydrogenating metal oxide catalyst.

8. The process which comprises catalytically hydrogenating a homocyclic hydroaromatic carboxylic acid having not more than two carbocyclic rings to a homocyclic hydroaromatic alcohol at a pressure of at least 10 atmospheres and at a temperature of 300-400 C. in the presence of a hydrogenating metal chromite catalyst.

9. The process of producing'homocyclic hydroaromatic alcohols, which comprises catalytically hydrogenating the acyl radical of a hexahydrophthalic acid at a temperature in excess of 200 C.

and a pressure in excess of 10 atmospheres.

10. The process of producing homocyclic hydroaromatic alcohols; which comprises catalytically hydrogenating a hexahydrophthalic acid at a temperature in excess of 200 C. and a pressure in excess of 10 atmospheres.

11..The process-of claim 10 wherein a hydrogenating metal chromite catalyst is used.

12. The process of claim 10 wherein a hydrogenating metal chromite catalyst is used, the temperature is 240 C.-400 C., and the pressure is 50- 400 atmospheres.

13. The process of producing homocyclic hydroaromatic alcohols, which comprises catalytically hyd'rogenating an ester of a hexahydrophthalic acid at a temperature in excess of 200 C. and a' pressure in excess of 10 atmospheres.

' 14. The process of preparing homocyclic hydro- I aromatic alcohols, the step which comprises reacting ethyl hexahydro-o-phthalate with hydrogen at a temperature ofapproximateiy- 385 C., 

